The present invention generally relates to apparatus and methods for magnetic resonance imaging (MRI), also known as nuclear magnetic resonance imaging (NMRI). More particularly the present invention relates to an apparatus having a near-field radio-frequency planar strip array antenna that can be used for parallel spatial encoded and for conventional series spatial encoded MRI. The present invention also relates to methods and MRI systems related thereto.
Magnetic resonance imaging (MRI) is a technique that is capable of providing three-dimensional imaging of an object. A conventional MRI system typically includes a main or primary magnet(s) that provides the background magnetic field Bo, gradient coils and radio frequency (RF) coils, which are used for spatial encoding, exciting and detecting the nuclei for imaging. Typically, the main or primary magnet(s) are designed to provide a homogeneous magnetic field in an internal region within the main magnet, for example, in the air space of a large central bore of a solenoid or in the air gap between the magnetic pole plates of a C-type magnet. The patient or object to be imaged is positioned in the homogeneous field region located in such air space. The gradient field and the RF coils are typically located external to the patient or object to be imaged and inside the geometry of the main or primary magnet(s) surrounding the air space. There is shown in U.S. Pat. Nos. 4,968,937 and 5,990,681, the teachings of which are incorporated herein by reference, some exemplary MRI systems.
In MRI, the uniform magnetic field Bo generated by the main or primary magnet(s) is applied to an imaged object by convention along the Z-axis of a Cartesian coordinate system, the origin of which is within the imaged object. The uniform magnetic field Bo being applied has the effect of aligning the nuclear spins, a quantum mechanical property of macroscopic particles comprising the imaged object, along the Z-axis. In response to RF pulses of the proper frequency, that are orientated within the XY plane, the nuclei resonate at their Larmor frequencies. In a typical imaging sequence, the RF signal centered about the desired Larmor frequency is applied to the imaged object at the same time a magnetic field gradient Gz is being applied along the Z-axis. This gradient field Gz causes only the nuclei in a slice with a limited width through the object along the XY plane, to have the resonant frequency and to be excited into resonance.
After excitation of the nuclei in the slice, magnetic field gradients are applied along the X- and Y-axes respectively. The gradient Gx along the X-axis causes the nuclei to precess at different frequencies depending on their position along the X-axis, that is, Gx spatially encodes the precessing nuclei by frequency (i.e., frequency encoding). The Y-axis gradient Gy is incremented through a series of values and encodes the Y position into the rate of change of the phase of the precessing nuclei as a function of gradient amplitude, a process typically referred to as phase encoding.
The quality of the image produced by the MRI techniques is dependent, in part, upon the strength of the magnetic resonance (MR) signal received from the precessing nuclei. For this reason an independent RF coil is placed in close proximity to the region of interest of the imaged object in order to improve the strength of the received signal. Such RF coils are sometimes referred to as local coils or surface coils.
There is described in U.S. Pat. No. 4,825,162 a surface coil(s) for use in MRI/NMRI imaging and methods related thereto. In the preferred embodiment of that invention, each surface coil is connected to the input of an associated one of a like plurality of low-input-impedance preamplifiers, which minimize the interaction between any surface coil and any other surface coils not immediately adjacent thereto. These surface coils can have square, circular and the like geometries. This yields an array of a plurality of closely spaced surface coils, each positioned so as to have substantially no interaction with all adjacent surface coils. A different MR response signal is received at each different one of the surface coils from an associated portion of the sample enclosed within the imaging volume defined by the array. Each different MR response signal is used to construct a different one of a like plurality of different images then being combined, on a point-by-point, basis to produce a single composite MR image of a total sample portion from which MR response signal distribution was received by any of the array of surface coils.
The use of a phased array RF coils or surface coils with a tuned and matched circuit including low impedance pre-amplifiers have been used to de-couple adjacent loops as a mechanism for improving the signal-to-noise ratio (SNR) and field of view (FOV). In this regard, it should be understood that the term xe2x80x9ccouplingxe2x80x9d refers to the coupling of an MR signal in one coil to an adjacent coil(s), such that the signal being outputted by the adjacent coil is a combination of the MR signal detected by the adjacent coil and the coupled MR signal. Consequently, the image from the adjacent coil would be distorted to some degree. Although overlapping adjacent coil(s) and using low impedance pre-amplifiers have been effective from the standpoint of improving SNR and FOV, such circuitry becomes ineffective when both the number of coils and the coil density is increased. In other words, as the spacing between adjacent coils and between adjacent portions of a coil is decreased signal coupling is increased irrespective of the tuned and matched circuits.
Although there are a variety of spatial encoding methodologies or techniques being implemented, the most popular method used in commercial MRI scanners is two-dimensional Fourier transform (2DFT) encoding in which a two-dimensional spatial plane (e.g., XY plane) is encoded with both frequency and phase of the MR signals. Typically during one data acquisition, only a one dimensional time-domain signal is obtained and thus 2DFT encoding requires repeating the data acquisitions sequentially to achieve a pseudo second dimension of the time domain signals. The second dimension of the spatial information is encoded into the phase component by repeating the data acquisition with different phase encoding gradient strengths (i.e., varying Gy to create the other pseudo-time dimension. In this technique, each slice of the imaged object is in effect divided into a multiplicity of layers in the Y-direction or axis corresponding to the number of pixels in that direction (e.g., 128, 256). The number of pixels in turn is representative of the desired image resolution, in other words the higher the resolution the higher the number of pixels. In addition, the X-direction scanning process or the data acquisition is repeated by sequentially and repeatedly stepping through each of these Y-direction layers. Because the resolution of the time-domain signals depends on the number of repetitions of the data acquisitions, and the repetition rate is limited by the MR relaxation times; a higher resolution image takes a longer time.
Two recent methods, the Simultaneous Acquisition of Spatial Harmonics (SMASH) imaging in the time domain and the Sensitivity Encoded (SENSE) imaging in the frequency domain, changes such sequential data acquisition into a partially parallel process by using a phased array, thereby reducing the scan time as compared to the sequential data acquisition technique. In these two techniques, it is recognized that the data sampled below the Nyquist sampling rate can be recovered if the sensitivity profiles of the phased array detectors can provide enough spatial information to either interpolate the data in the time domain or unwrap the data in the frequency domain.
The time domain method recognizes the equivalence between phase-encoding with MRI gradients and the composite spatial sensitivity inherent in the detectors. It uses a numerical fitting routine to, among other things, interpolate a decimated number of phase-encoding steps and thus, achieve reductions in scan time. Although this numerical approach was instrumental in demonstrating the original SMASH concept, it did not recognize the underlying analytical relationship between the weighting factors for the composite harmonics, the FOV, the spacing of the detectors, the harmonic orders, and the sensitivity profiles of the detector coils.
Another problem is that conventional MRI phased array coils are unable to deploy a large number of coils due to the limitations imposed by both their loop structure and the de-coupling requirements for the mutual induction between the elements. Because the number of coils in the phased array corresponds to the maximum decimation factor for reducing the number of phase encoding steps, existing phased array designs significantly limit the potential for parallel spatial encoding.
It thus would be desirable to provide an array antenna or RF MR signal detector, as well as systems and methods embodying such an antenna/detector, that would allow the number of antenna elements for detecting near filed signals in the array antenna to be increased while maintaining the de-coupling of adjacent antenna elements and the desired SNR and FOV characteristics. It would be particularly desirable to provide such array antenna/detector, system and method in which the array antenna includes a multiplicity of elements for detecting MR signals, such as 6 or more, 32 or more or 64 or more elements so as to significantly reduce scan time. Such an array antenna/MR signal detector preferably would be simple in construction as compared to prior art antennas and/or MR signal detectors.
The present invention features a device for detecting or receiving electromagnetic signals and more particularly a device for detecting magnetic resonance (MR) signals from excited nuclei. Also featured are a detection apparatus embodying such a device, an MR excitation and signal detection apparatus embodying such a device, a magnetic resonance imaging (MRI) system embodying such a device and/or such an MR excitation and signal detection apparatus. Further featured are methods related to the above-described detection device, apparatuses and system.
The detection device includes a plurality of conductors being arranged so as to be parallel to each other. Each of the conductors has a length that is set so as to substantially reduce the coupling of a signal in one of the conductors to an adjacent conductor. More particularly, the conductor length is set so as to be equal to be about nxcex/4, where n is an integerxe2x89xa71 and xcex is the wavelength of the signal to be detected. For MRI applications, xcex is the wavelength corresponding to the NMR resonance frequency for the nuclei being subjected to a given magnetic field strength by the main or primary magnetic coils. For example, the quarter wavelength for a proton NMR in a 1.5 Telsa magnetic field in air is about 117 cm.
In a specific embodiment, the detection device includes a multiplicity of parallel conductors, more particularly, about four or more conductors. In another specific embodiment, the detection device includes sixteen (16) or more conductors, more particularly thirty-two (32) or more conductors. In a further specific embodiment, the detection device includes a multiplicity of conductors in the range of from about 4 to about 32 conductors, more particularly in the range of from about 4 to about 16 conductors and further in the range of from about 16 to about 32 conductors.
The detection device further includes an encapsulation member comprising a substrate, on one surface of which is disposed the plurality of conductors and the other opposing surface thereof is disposed a ground plane, and an overlay that covers the conductors disposed on the substrate. The substrate and the overlay are made of a material having a dielectric constant so the wavelength of the electromagnetic wave on the each conductor is reduced so as to be in a desired range for purposes of scanning. In an exemplary embodiment, the encapsulation material has a dielectric constant in the range of from about 6 to 9.6 such that the quarter wavelength for a proton NMR in a 1.5 Telsa magnetic field on each encapsulated conductor is in the range of from about 48 cm to 38 cm. The ground plane is an electrical conductive material including, but not limited to, copper, aluminum or silver. The ground plane, as is known to those skilled in the art, is applied or otherwise secured to the other opposing surface and to keep EMF on strip in a quasi-TEM mode.
In other specific embodiments, the detection device is configured to operate such that the electromagnetic (EM) wave on each of the conductors is one of a standing wave or a traveling wave. In the case of a standing wave, the detection device further includes a mechanism that terminates one end of each conductor as one of an open circuit or a short circuit. In the case of a traveling wave, n is an even number such that the length of each conductor is a multiple of xcex/2 and a termination of each conductor is a resistive match.
Additionally, the detection device can be configured to include an electromagnetic field (EMF) interference guard that is electrical connected in a fashion so that the guard electrical isolates at least a portion of each conductor (e.g., the ends of the conductors) to minimize the EMF interference to the plurality of conductors by/from the environment including areas of the imaged object outside of the specific area being scanned. In one embodiment, the EMF interference guard includes two guard strips, each guard strip being disposed proximal the ends of each conductor strip and so as to minimize EMF interference from environmental electromagnetic waves approaching the ends of each conductor. In a more specific embodiment, a long axis for each of the guard strips extends generally perpendicular to a long axis of each conductor.
In another embodiment, a plurality or more of guard strips are arranged about the periphery of the plurality of conductors so as to, in effect encircle the ends and sides of the plurality of conductors. In yet another embodiment, the EMF interference guard comprises one or more members that are configured so as to be provide any of a number of geometrical shapes such as circular and oval. These one or more members are formed about the ends or about the periphery of the plurality of conductors such that the configuration minimizes external EMF interference. In a more specific embodiment, the EMF interference guard or each guard strip thereof is electrical connected to ground.
According to another aspect of the invention, the plurality of conductors are arranged so as to provide both narrow band de-coupling, as described above, and broadband de-coupling of the conductors. In specific embodiments, such broad-band de-coupling can be realized when the spacing (xe2x80x9csxe2x80x9d) between the conductors and the height (xe2x80x9chxe2x80x9d) of the encapsulation member is set so a ratio s/h is greater than or equal to about 3 or about 2.5.
Also featured are a detection apparatus including a detection device, having a plurality of conductors as described above, and a plurality of receivers, one receiver for each conductor. In a more specific embodiment when using the detection apparatus in an MRI system, the detection apparatus further includes a plurality of transmit/receive (T/R) switching mechanisms (e.g., switches) one for each conductor/receiver path. The T/R switching mechanisms are configured and arranged so as to de-couple each detection device conductor from its corresponding receiver during the period of time when an excitation electromagnetic signal is being generated to depress the magnet moment of the nuclei within the imaged object and also to couple each detection device conductor with its corresponding receiver during the time period when the excitation electromagnetic signal is not being generated, such as when a MR signal is to be detected.
Additionally there is featured an excitation and detection system including the features described above for the detection system. Such an excitation and detection system further includes a transmitter that generates the excitation electromagnetic signals or pulses, an antenna for transmitting these signals into at least the region of the imaged object to be scanned, and a controller that selectively controls signal transmission and signal reception so that each occurs at predetermined times and/or predetermined time intervals.
Further, there is featured a MRI system including the excitation and detection system described above, a main or primary coil that generates a homogeneous magnetic field in a predetermined region in which the object or portion thereof is to be imaged, gradient coils for generating one or additional magnetic fields, controllers for controlling the operation and energization of each of the main and gradient coils, the generation/transmission of the excitation electromagnetic (RF) signals and the acquisition/detection of MR signals by the detection system. Such a system further includes a processing apparatus that processes the data acquired so as to yield an image of the object that was scanned.
Other aspects and embodiments of the invention are discussed below.